Signaling system



March` 16, 1943..

A. A. LuNDsTRoM v -S IGNALINGl SYSTEM Filed Dec. l0, 1940 2 Sheets-Sheet lA AIA .SEL

XN @Px Enigma u @EPEE l I I L 2l sheetsfsheet 2 A. A. LUNDsTRoM SIGNALING .SYSTEM Filed neg. 1o, i940 March 16, 1943.

)NMEA/ron By .A.LUND$7`OM @m'aa ArToR/QEV Patented Mar. 16, 1943 SIGNALIN G SYSTEM Alexis A. Lundstrom, East Orange, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 10, 1940, Serial No. 369,391 13 claims. r(ci. 179-18) This invention relates to signaling systems and particularly to the receiving equipment of a transmission medium over which alternating current signals, preferably within the voice fre'- quency range, are transmitted. The received signals are then used for any desired purpose such as, for instance, the operation of controlling equipment for the selective positioning of telephone or telegraph switching apparatus or as vehicles of intelligence which is to be recorded or retransmitted to a more distant point.

In systems of this kind, the receiving circuit usually consists of a channel for each frequency in the signals and comprises a filter and adetecting device the latter of which is responsive to the signal energy that gets through the filter.

A registering or recording device which is conwhich are of the greatest intensity for the filtersadjoining either side of the one through which the current Will pass and progressively diminishes in intensity with respect to the other -lters more remotely located. If a detector-such as a vacuum tube is connected to the output side of each of the filters, the detector connected to the filter which passes the current will, of course, operate. There is danger, however, that the detectors connected to the filters on either side of the one which responds to the current will also operate on the current produced by the transient disturbance set-up in the adjoining filters at the beginning and end of the pulse. To avoid this occurrence, it is the usual practice in carrier multiplex telegraph systems, for example, to reduce the intensity of unwanted transient by providing a shaping filter Afor each `signal frequency at the transmitting station. Although with this arrangement a disturbance is, of course, produced in the band-pass filters 'of the receiving station at the beginning and end of the shaped signal current, this disturbance in the filters on either side of the filter which passes the incoming signal current is'suii'iciently attenuated by thev particular shaping filter to prevent the false operation of the vacuum tube detectors associf ated with the adjoining filters."

The development of the gas-filled tube, isa vessel containing one orl more of the S0- called inert gases at low pressure, is normally which non-conducting until a breakdown potential is applied to two of its electrodes, is thereafter conductive on a voltage less than breakdown voltage and can be only quenched thereafter by 'removing or momentarily reversing the positive potential connected to the anode, has made it possible to take advantage of certain electrical properties of band-pass filters by which substantial economiesA can be effected in signaling systems ofthe carrier'multiplex type through the elimination of the shaping filters at their' transmitting stations. Thus it is known that lif a plurality of paralleled band-pass filters are connected to the receiving end of a transmission yline, and these lters all have the same phase shift characteristics in their pass-bands, are adapted to pass equal steady band Widths in cycles and are equally spaced in cycles within the ventire available frequency` range, a signal impulse of two or more frequencies applied simultaneously to. the transmission line will cause the wanted frequencies out of the filters to reach va maximum intensity at the same time that the unwanted frequencies, produced'by the transient kspectrum in the adjoining filters, will reach a minimum. In the absence of shaping filters at vthetransmitting station this phenomenon will bepresent, of course, in the filter network of the receiving station since the filters thereof are ordinarily (though not necessarily) of equal sections and, therefore, of substantially identical phase characteristics and would be equally spaced within the frequency range. However, due to the use of the ordinary vacuum tube detector `on the output side of each of the filters ofthe receiver, the detectors which are on either side of the filter which passes a signal frequency,V would be subject to false operation on the transient'spectrum at the beginning and end of the signal frequency pulse and this is avoided, as; before stated, by using shaping filters at the transmitting station, Vwhich insure a minimum spectrum'effect on the output side of the filters of the receiver. f

However if, in a carrier multiplex receiver, the vacuum tube detectors thereof are replaced by gas-filled tube detectors and means are provided vfor controlling the time when these detectors are to berendered conducting to respond to'inco'ming signal frequencies and the length of time duringwhichtheir conductivity is to be sustained, it is evident that the use of filters ofiden'tical phase characteristics equallyr spaced,

altogether the use of filters at the transmitting station since the gas-filled detectors of the receiver can then be kept unresponsive to the unshaped incoming signal frequencies until the maximum intensity of the unwanted frequencies produced on the output side of the receiving filters consequent to the. application of the signal frequencies to the input side of the filters, will have been reached and passed, subsequent to which the detectors can be kept conducting for a measured duration, cutting ofi such conductivity before the termination of the signal frequencies so as to again avoid false operation of the detectors by the unwanted frequencies produced by the transient spectrum at the termination of the signal frequencies, `And since, if the receiving filters are of identical phase characteristics and are equally spaced within the available frequency range, the unwanted frequencies will all reach their maximum yintensities at the same time regardless of which of the receiving lters are operated by the incoming signal frequencies, it follows that one common means may be utilized to control the time when the detectors shall be rendered conducting as well as the duration of such conductivity. 4

In accordance with a feature of this invention, therefore, each channel detector of a signaling system is provided with a filter network of .equal sections and equal band width to all the others and means are provided whereby the incoming signal energy operates a circuit network herein called the enabler which is adapted to activate the cha-nnel detectors after a time interval subsequent to the arrival of thc signal energy and which is of sufiicient duration to permit the dissipation of the transient disturbance. This time interval is determined by the physical constants of VVthe .filter network and the width of the pass band which, being substantially the same for all filters, .produces the same .time interval in each of the channels.

This and other'features of the .invention will be more readily ascertained from the .following detaileddescription of .said invention,"appended claims and vattached drawings 1in which:

Fig'..1 is a conventional representation of the :fundamental elements. of the invention in its generalized form l Fig. 2 showscertain graphic-illustrations in- .tended to show the V.character ofthe disturb-ance in the filter receiving .the signal .as well as in the filters receivingthe transient disturbance ,in-

duced thereby; while such as a telephone line, radio channel, carrier band, etc. and in which `a commutatoror keying 4mechanism is utilized to apply a plurality of frequencies simultaneously to the medium in one signal pulse, and a receiving station having as many channels as therev are signal frequencies, each channel being provided with a band-pass =filter device Ji-fnvhaving uniform phase -shift characteristics, equal band widths in cycles of which the signal frequency constitutes the rmidband frequency, and a detector I-V-N- for veach CII channel operatively connected, respectively, to each of the filters and responsive to the signal frequency passing to the output side of the lter. The receiving station is further augmented by a common untuned detector circuit or enabler which operates on the reception of the signal at the input to the band-pass filters. This operation initiates a relay timing chain which utimately causes all the detectors associated with the filter outputs to be simultaneously sensitized after a particular time interval when the intensities of the wanted frequencies out of the filters are a -maximum while the intensity of the unwanted transient spectrum frequencies out of adjoining filters are a minimum.

The operation of the enabler in avoiding the sensitization of the'detectors until after the dissipation of the spectrum transient can best be explained by diagram, reference being had to Figs. 2X, ZY and 2Z which represent the results of actual tests. For simplicity consider a twofrequency system using the frequencies fA and ,fB and a receiver equipped with band-pass filters A and .B having the'mid-band frequencies fr. and je. When a square pulse fA .appears at the input to these filters as shown by Fig. 2X, the output of filter A is roughly shown by the envelope indicated by Fig. 2Y `as building up to a maximum value at a subsequent time T2+T1. The unwanted current output of filter B (due to the transient disturbance set up by the impact of A on filter B) 'rises to a maximum value after van interval T2 and Ithen decays to a minimum value after interval Tz-i-Ti as indicated by the lefthand graph von Fig. ZZ. Provided that the channel detectors are sensitized momentarily by the enabler at the time T2-{-T1,the largest ratio between the wanted output of filter A and the unwanted output of filter B is achieved witha minimum in delay between the first appearance -of currents out of the filters and the time that the filters lare sensitized.

Referring aagin to Fig. 2Z, it is seen that the unwanted current after the first maximum de- Acays in a succession of maximums and minimums while the-end of the applied `pulse sets up another transient current which is in the same order of magnitude-as the initial transient and has the same envelope with Va minimum occurring at the time (Tz-l-Ti) after the disappearance lof the applied frequency fA. It is obvious, therefore, that the sensitization of the detectors, while it must Vbe rendered effective lafter a time T2+T1 from the appearance of the'frequency fA at the output of filterA in order to avoid the false operation of thedetec-tor connected to lter B, it must be equally renderedineffective bythe time that the decay `of frequency fA begins in order'toavoid, at the end of the pulse, a false operation of the detector connectedf to filter B.

The time T2 is the envelope delay through the filter and is due to the .slope of the phase shift vversus frequency characteristic of the 'filter pass band. This simply displaces the pulse along the timescale to the extent of time T2, and for any particular band-.pass configuration `with a fixed number of sectionsyit is inversely proportional to the frequency band Width of the filter in cycles. From which it appears evident that, in order to obtain the same time T2 in each of the channels, the filters'therein will have to have substantially identical phase shift characteristics. The time T1 is due to the restricted frequency pass-band of one `filter and is equal tothe reciprocal of the band width in cycles. `Wlience it is further evidevice herein called a sender.

dent that, in order to obtain the same timeq'Ii in `each of the channels, the filters therein will have to have the same band width in cycles. In order, therefore, to sensitize all the detectors in all the channels at the same time, the lter envelope delay due to the phase shift of the several lters plus the build-up time out of the filter due to the restricted pass band shall be the same for all the filters in the receiver.

Assuming a band of frequencies available for signaling it is necessary to spot the signaling frequencies over this range so that they will provide a maximum discrimination between the wanted (see Fig. 2Y) and the unwanted filter responses (see Fig. ZZ). VThe intensity of the transient response is, to a good approximation, proportional to the lter band width and inversely proportional to the difference in cycles between the filter mid-band frequency and the frequency applied outside of its band. In order to use the available band to best advantage, therefore, it is necessary to make the unwanted transient response in adjoining filters equal and a minimum, and this may be achieved by imposing the additional requirement that the signaling frequencies shall occupy the entire availe able frequency range and be equally separated in cycles. y

Referring, now, to Fig, 3 which represents a practical application of the invention to an automatic telephone system, X represents a transmitting station equipped with a device I suitably arranged to produce one or more currents of different frequencies, preferably in the Voice frequency range, in response to operations thereof for designating digits or other characters making up the signal to be transmitted; 2 represents a line extension between said station and a central point, which extension may be established by any suitable means such as, for instance, line extending switches and the like whence, for the purpose of switching control and the further extension of the transmitting station X to the receiving station Y in the event that ysaid two stations are not connected by a direct line, the line extended from station X may be connected, through a selectable sender connector relay I3, to a switching control l The sender comprises coupling transformers 3 and 4, volume limiter II (if one is'necessary), the enabler oircuit II), and the six signal receiving channels A-E'it being assumed that there are six signal frequencies equally spaced in cycles over the en- .tire band in cycles. Each channel comprises a band-pass filter such as JA, a coupling transformer such as Ta, a three-element cold cathode gas-filled tube such as Ga one of whose control electrodes is connected to a terminal of the secondary winding of transformer Ta and the other (in parallel with corresponding electrodes of the remaining tubes) to a contact of relay I5, and a resistance such as Ra bridged across the control electrodes of the tube and likewise paralleled tothe contact of relay I5. The volume limiter I I, the band-pass filters fA-fr, the gas-,filled tubes Ga-Gf and other electrical and electromecInanical imits herein schematically indicated are commercially available pieces of apparatus which need no further description of their physical characteristics. The enabler I comprises the amplifier tube 8, the rectifier bridge 5 and the polarized relays I4, I5, AI6 and I'I with their intermediate capacitor-resistance networks controlling the operating timesof said relays in themanner described hereinafter. It will be noted at this time, however, that relays I4 and I5 have a permanent series biasing circuit through their lower and upper windings respectively which causes their armatures to assume the positions shown in the drawing, while relays IS- and I1 have independent biasing circuits through their respective upper windings. In the case of relay I6, there is an operating circuit also normally established from battery through the lower winding of said relay, resistance 1, armatures and contacts of relays I4 and I1, respectively, to ground. The currents through both windings of relay I6 oppose each other, the position assumed by the armature of the relay as engaging the left contact being due to the fact that the current through the lower winding overpowers that owing through the upper winding. The armature of relay I1, on the other hand, engages its left contact solely in response tothe current iiowing through its upper or biasing winding, the circuit path for the lower winding being open at the right contact of relay I6.

InA the operation of the invention as applied to a signal system in which the connection between station X and station Y is to be established by means of switching selectors controlled by a sender responsive to the alternating currents transmitted from device I at station X, the trans-f mission line 2 between station X and the sender may be established in any suitable manner, say through the medium of a relay I3, where a sender is available to a plurality of transmitting stations to each of which it may be connected by switching circuits well known in the automatic switching arts, relay I3 being merely representative of the connecting medium in such a circuit for connecting the line to the sender. Where the invention is adapted for use with Acarrier multiplex telegraph systems, for instance, the line may well terminate directly at the receiving station Y in which event the line conductors 2 are extended directly to the input side of transformer 3.

In transmitting the desired signals from station X, the sending device I may be operated to produce two or more'of the frequencies which makeup the signal to be transmitted, which frequencies are, of course, the same as those for which the channel detectors A-F are adaptedY for response. v

The resulting current thus produced by the operation of device I is transmitted over a circuit which is traced from one side of the sending device I, lower conductor of the looped pair 2, lower back contacts of relay I2, next to the inner contacts of relay I3, primary winding of transformer 3, inner contacts of relay I3, top back contacts of relay I2, upper conductor of looped pair 2, to theother side of the station sending device I. The current, composed of the signal frequencies, is induced in the secondary winding of transformer-3 and fed into the input side of the volume limiter II whence' apart of the energy is applied, through the secondary of transformer 4, tothe grid of tube 8 for amplification therein while the remainder is applied through'a suitable resistance pad (not shown) to the band-pass lteIS fel-fb;

The output of tube 8 is appliedl to the two horizontal terminals of rectifier bridge 5 from which, throughthe two vertical terminals of said bridge, it is further applied serially through the upper and lower windings of relays I4 and I5', respectively, whereupon the current owing through these respective windingsv is sufficient to oppose, that normally flowing through their respective biasing windings 'and cause both relays to operate. Relay I applies solid ground to the lower control electrode .of all the channel tubes Gir-Gr, to the vupper control electrodes of said tubes via high resistance Ria-Rf and to one side of the secondary windings .of the kchannel transformers Ta-T'f via the high resistances Ria- Ri and another low resistances in series with each of said high resistances.

When the armature of relay I4 breaks with its contact, a charging circuit is -established for normally discharged condenser Il which traces from ground through said condenser, resistance 1, lower winding of relay I6 to battery. Since, with the operation of relay IA, the circuit through the lower winding of relay i6 is yopened andthe charging circuit of condenser 6 is established through said lower winding, and since, with no current flowing through said lower winding, the

armature of the relay would come into engagel 'ment with the right contact as a result of the rcurrent through the upper winding, the effect of the condenser charging current is to prolong the time during which the armature of relay I6 engages its left contact until the magnetic effects i v of the decaying charging current through the lower winding of relay I6 is no longer sufficiently powerful to overcome the elects of the normally existing biasing current through the upper winding of said relay, at which time the relay operates and the armature thereof moves into lengagement with its right contact, thereby establishing the charging circuit of condenser I8 over a path which traces from ground through condenser I8, armature and right contact of relay I6,'resistance I9, lower winding of relay I1V to battery. This charging current is in the direction to oppose the eiTect of the biasing current normally flowing through the upper winding of relay I1 as above described so that, during the condenser charging period the armature of relay I1 moves from its left contact into engagement with its right contact whereupon ground is connected to the upper control electrode oi the channel tubes Ga--Gf via conductor 20, the secondary winding of each of the transformers Tit- Tf and each of their respective associated low resistances connected between the -upper terminal of the secondary winding of each of said transformers and the upper control .electrode of the associated tube. Since both control electrodes of each of the tubes are now connected to ground, that portion of the signal energy which passed through the band-pass lters fa-Jf involved for the frequencies carrying the signal energy will now cause the potential of the signal to be applied to the two control electrodes of the particular channel tubes and cause the gap therebetween to break down and become conducting. Because of the partial ionization of the gaseous substance that results from the breakdown of the control gap within each of the tubes to which the signal voltage is applied, the potential required to break down the main gap between the anode and cathode electrodes is reduced to the amount required which is below the value that would be required if the gap between the control electrodes had not been rst ionized. Upon the breaking down of the main gap, a circuit is established for each channel relay Sa--Sf for which the associated tube has become conducting through its control and main gaps, a typical one of said circuits being traced, for instance, from positive battery 2l, winding of relay Si, main gap electrode to lower control electrode oftube Ga, conductor 22, to ground on the armature of relay I5. Relay Se operates in said-circuit andcloses through one or more sets by other relays in the group of relays Sa-Sf responsive to associ-ated tubes rendered conducting in ythe manner above described, register the digit or character represented by the particular combination of frequencies in the transmitted signal.

When condenser IS is fully charged, the eurrent` through the lower winding of relay I1 ceases to v-liow and the armature of this relay now assumes its former position of engagement with the left contact under the influence of the current flowing through theupper or biasing winding. In sodoing, ground is thereby removed from conductor 20 and, therethrough, from the upper control electrode of all the channel tubes Cia-G1, thereby rendering further duration of the signal impulse impotent to inuence any further operation. However, since the tubes which correspond to the frequencies in the signal have already operated and are maintained operated via their respective anode circuits and the contacts of relay I5 independent of the frequency composition of the signal impulse, the removal of the ground from the upper electrode of all tubes will have no other efect except the desirable one of preventing tubes which have not already operated from operating for any cause whatsoever.

When the signal impulse terminates, relays I4 and i5 release, condenser E5 immediately discharges to ground potential via the armature and left Contact o relay I4 and left contact and armature of relay I1 to ground, while current is simultaneously reestablished through the lower winding ci relay Et, causing the armature of lthis relay to assume it normal position of engagement with its left contact. Condenser I3 is discharged thereby. Relay i5, in releasing, opens the main gap circuit Vof the operated tubes causing the same to quench and release the corresponding channel relays, thereby ,placing all channel detectors in a condition of receptivity to the next incoming signal. The incoming signals which are successively registered in the register relays of the-sender are then utilized to selectively position a train of switching selectors and connectors for extending the transmitting station 'X to the receiving station Y, after which the sender is disconnected the line by the release o relay i3 and the two stations are connected together by the operation of relay I2, all in the manner described in my copending application above mentioned.

While l have described my invention and the means for utilizing the same in connection with its application to a particular kind of a transmission line, it is to be understood that various other 1applications and embodiments thereof maybe Vmade by those skilled in the art without 'departure'irom the spirit of the invention as defined within the scope of the appended claims. Thus if the invention were to be adapted for application to a carrier multiplex telegraph system, the signal receiving channels A-l'l might form an part of the transmission line itself thereby dispensing with the use of a sender, while the receiving station Y would comprise equipment responsive tothe relays Sa-Sf which would be set in accordance with the frequencies comprising each of the signals either for decoding and permanently recording the same or for registering them for retransmission to some' passed through said lter devices, and means re sponsive to a signal impulse for sensitizing all of said electronic means for response to said impulse,

. 2. A receiving circuit for l,a signaling system adapted for transmission of plural frequency signals thereover comprising a filter device for each signal frequency, said devices having substantially identical phase characteristics and substantially equal band-pass widths and being equally spacedwithin the entire signal frequency band, gas-iilled electronic means in circuit with each of said lter devices normally unresponsive to currents passed through said filter devices, yand means responsive to a signal impulse for sensitizing all of said electronic means for response to said signal impulse.

3. A receiving circuit for a signal system adapted for the transmission of plural frequency signals thereover comprising a filter device for each signal frequency, said devices having substantially identical phase characteristics and substantially equal band-pass Widths and being equally spaced Within the entire signal frequency band, gas-filled electronic means in circuit with each of said iilter devices normally unresponsive to currents passed through said filter devices, and means responsive to a signal impulse for sensitizing all of said gas-filled electronic means for response to said signal impulse a predetermined interval subsequent to the application of said signal impulse to said filter devices.

4. A receiving circuit for a signal system adapted for the transmission of plural frequency signals thereover comprising a iilter device for each signal frequency, said devices having substantially identical phase characteristics and substantially equal band-pass Widths and being equally spaced Within the entire signal frequency band, gas-filled electronic means in circuit with each of said filter devices normally unresponsive to currents passed through said filter devices, and means responsive to a signal impulse for sensitizing all of said gas-filled electronic means for a time duration measured between a predetermined interval subsequent to the application of said signal impulse to the input sides of said filter devices and a predetermined interval prior to the termination of said i signal impulse at the output sides of said filter devices.

5. In a signaling system, the combination with Y a transmission medium, of a plurality of receivnal frequency band, and means responsive to said signal .energy for activating all said receiving means for response to said signal energy a predetermined interval subsequent to said energy being available at the input sides of said filter devices.

6. In a signaling system, the combination With a transmission line of a plurality of receiving means connected thereto for response to plural frequency current signals transmitted thereoverl but held normally unresponsive to said signals, comprising filter devices of equal phase shift characteristics and equal frequency band Widths for said receiving means, and means responsive to `said' nsignals for activating said receiving means -aninterval after each signal is applied' to'thc'input sides of said filter'`v devices which is.

determined by the phase shift and band Width properties of said filter devices.

7. In a signaling system, the combination with a transmission line of a plurality ofreceiving.

means ,connected-thereto for response to plural frequency current signals transmitted thereover, each of said receiving means comprising a filter section'of equal phase shift characteristics and 'equal band width in cycles to corresponding filter sections in each of the other of said receiving means, a gas-lled electronic device normally unresponsive to the current passed by said filter section, and means responsive to saidvsignal for rendering each' of said' gas-filled electronic devices partially conductive a predetermined interval after a signal is applied to the input side of saidiilter devices, whereby the gasfilled electronicdevice in each of said receiving means through, which a frequency of the signal hasl passed becomes Wholly conductive in response thereto.

8. In a k signaling system, the combination with a transmission'linev of a plurality ofv receiving,

means connected thereto for response to plural frequency current signals transmitted thereover,v each of said receiving means comprising a filter section of equal phase shift characteristic and equal bland Widths in cycles to corresponding filter sections in each of the other of said receiving means, a gas-filled electronic device normally unresponsive to the current passed by said filter section, means responsive to said signal for rendering all of said gas-filled electronic devices partially conductive a predetermined interval subsequent to the application of said signal to the input sides of said filter devices, Whereby the gas-filled electronic device. in each of said receiving means through which a frequency of the signal has passed becomes Wholly conductive in response thereto, and means for restoring those gas-filled electronic devices Which have not been rendered Wholly conductive.

9. In a signaling system, the combination with a transmission line of a plurality of receiving channels connected thereto for response to plural frequency current signals transmitted thereover, each of said receiving channels comprising a filter section of equal phase shift characteristic and equal band Widths in cycles to corresponding iilter sections in each of the other of said receiving channels, a gas-filled electronic device normally unresponsive to the current passed by said filter section, means responsive to a signal for rendering all of said gas-filled electronic devices partially conductive a predetermined interval subsequent to the application of said signal to the input sides of said lter devices, whereby the electronic device in each of saidreceiving. channels. through. which a component frequency of the signal'. has passed be.-

comes wholly conductive. in response thereto,

response to a plural' frequency current signal.

transmitted over saidline a predeterminedinterval subsequent to the application ofsai'dlsig.- nal to the input sidesof'saidlter sections, said means comprising a rst pair; of polarized. relays responsive to said signal. for applying a. po.- tential to a control electrode. of each of said gas filled conductor devices, a second'pair of" polarized relays for applyingl av potential to another control electrode of each of said gas-filled conductor devices whereby all ofr said devices are rendered conducting between their respective pair of control electrodes, and capacitor-resistor networks interconnecting saidfl'rst and said second pair of polarized relays for operating said second pair of polarized relays after a first predetermined interval' and for. keeping them operated for a'second predetermined interval..

11. In a signaling system,` the combination with a transmission line and a plurality of channel detectors each comprising a filter section ofV ductor devices partially conductive. for response.

to aplural frequency current signal transmitted over saidline a predetermined interval` subse-A quent. tol the application. of said signal to the input sides of said filter sections, said means comprising a rst pair of polarized relays. responsive to said signal for applying a. potential to a control. electrode of each. of said gaslled. conductor devices, a second. pair of polarized` relays for applying a, potential. to. another control electrode. of each. of said gas.- filled.V conductor devices whereby all. of said devices are rendered conductivebetween their respectivepair of control electrodes, and capacitor-resistor networks interconnecting saidV first and said secondpair of polarized relays for operating saidsecond pair of polarized relays after a first predetermined interval measured by the duration of said peak disturbance and for keeping` themY operated' for a second predetermined interval..

12. In a signal selective system, the combina.- tion with energy responsive means, of frequency discriminating means and time discriminating means for activating said energy responsive means a predetermined interval after a plural frequency signal is applied to said frequency discriminating means.

I3'. In a multifrequency synchronous signal transmission system' comprising a selecting receiver channel for each frequency in a plural frequency signal, means for causing the Wanted currents ineach frequency of said selecting receiver channel to rise to a maximum at the same time that the unwanted transient currents in adjacent channels are a minimum, said means comprising band-pass filters of substantiallyv ALEXIS A. LUNDSTROM. 

